Estimation on Removal of Nitrogen and Phosphorus through Fisheries of Gracilaria verrucosa at an Estuarine Lagoon, Lake Nakaumi, Japan

Abstract

To estimate the role of macrophyte fisheries in controlling water quality in eutrophic lakes, nitrogen and phosphorus contents of macroalgae from an estuarine lagoon, Lake Nakaumi, Japan were analysed. Nitrogen and phosphorus contents of edible algae, Gracilaria verrucosa, were 3.1 and 0. 21% dry w, respectively. The amounts of nitrogen and phosphorus removed from L. Nakaumi through fisheries of 551 t dry w of G. verrucosa in 1960 were therefore estimated to be 17 and 1.2 t, respectively. These amounts were equivalent to 0.56 and 0.41% of present annual loads of nitrogen and phosphorus into Lake Shinji, River Ohashi and L. Nakaumi. An increase in the fisheries of G. verrucosa of up to 5 times the 1960 yield would result in removal of 2.8 and 2.0% of nitrogen and phosphorus loads to the lakes and the river. This may be possible because the fisheries yield of G. verrucosa in 1960 was estimated to be only 13% of the total annual growth.